R600: Proper insert S_WAITCNT instructions

R600: Proper insert S_WAITCNT instructions

Some instructions like memory reads/writes are executed
asynchronously, so we need to insert S_WAITCNT instructions
to block before accessing their results. Previously we have
just inserted S_WAITCNT instructions after each async
instruction, this patch fixes this and adds a prober
insertion pass.

Patch by: Christian König

Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Signed-off-by: Christian König <deathsimple@vodafone.de>
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172846 91177308-0d34-0410-b5e6-96231b3b80d8